Current state-of-the art positron emission tomograph (PET) units employ a plurality of blocks of bismuth germanate, typically having array of a multiplicity of detector crystals. A multiplicity of these blocks are held in a gantry surrounding a volume into which items are inserted for analysis. Mounted to each of these blocks are an array of light-sensitive detectors, such as photomultiplier tubes (PMT). The use of reflectors between each of the crystals confines the light generated from a gamma ray interaction, and light guides control the distribution of light to the PMTs making it possible for each crystal to be identified by a unique set of PMT signal combinations.
In a normal utilization of the PET unit, a radiopharmaceutical is introduced into the subject for analysis such that positrons are created in the course of the decay of the radioactive isotope. Upon emission, the positron encounters an electron and both are annihilated. As a result of each annihilation, gamma rays are generated in the form of two photons. As is well known, these two photons are emitted in approximately opposite directions from one another. The precise position of the positron-emitting isotope can be determined by surveying these photons by determining the light output of the various crystal detectors in the gantry surrounding the object under study.
Prior to the initial use of a PET unit, the detectors thereof must be calibrated such that subsequent use can provide the needed accurate situs of the positrons. This is conventionally accomplished using a gamma-ray source of about 511 keV positioned within the unit such that the multi-crystals of a given detector block are substantially uniformly subjected to the radiation. Usually the detector units are operated in a coincidence manner in order to ascertain true radiation from the source in contrast, for example, to scattered radiation. Typically, the gamma-ray source is .sup.68 Ge having an energy of 511 keV. Other sources in this same range of energies are typically .sup.22 Na and .sup.18 F. The distance to the crystal faces is sufficiently large that the source can be considered a "flood source" in that the radiation is uniform across the face of the multi-crystal detector block. The output of the bank of PMT's is monitored to determine that the light output from the various crystals of the block is substantially uniform, and that there is uniformity between light output of each of the blocks. Such factors as orientation, cut depth, photomultiplier tube response, etc. can affect the uniformity. Of course, a PET unit might require periodic re-calibration of the detectors.
In an effort to obtain better spatial resolution of the situs of the origin of the positron, it has been proposed that block detectors could be modified by increasing the number of crystals per block to 144 (a 12.times.12 array) and 256 (a 16.times.16 array). This is thought to permit obtaining detection capabilities in smaller segments around the gantry. While such capabilities have been encouraging, when calibration was attempted using conventional methods, there was excessive deviation in light response across the detector face, particularly in the edge crystals.
Therefore, it is an object of the present invention to provide an improved method for the calibration of multi-crystal block radiation detectors wherein substantially uniform light output is achieved for all crystals of the block.
It is another object of the present invention to provide a method for the calibration of block radiation detectors having in excess of about 64 crystals per block whereby uniform light output is achieved for all crystals of the block when the block has been properly fabricated.
A further object of the present invention is to provide a method of calibrating multi-crystal block radiation detectors with a gamma ray energy in excess of about 0.7 meV.
These and other objects of the present invention will become apparent upon a consideration of the following detailed description of the invention when taken together with the illustrations within the figures.